This invention relates generally to injection molding and more particularly to an improved melt conveying manifold having provision for cooling as well as heating.
As is well known in the art, multi-cavity hot runner injection molding systems have a heated manifold to convey the pressurized melt from the inlet from the molding machine to a number of outlets, each leading to a heated nozzle which extends to a gate to the respective cavity. These manifolds have various configurations depending upon the number and arrangement of the cavities. Different arrangements of electrical heaters are known for heating the manifolds to the necessary operating temperature. For instance, the manifold can have an electrical heating element integrally cast into it as described in Gellert U.S. Pat. No. 4,688,622 which issued Aug. 25, 1987. Cartridge heaters can be cast in the manifold as disclosed in Gellert U.S. Pat. No. 4,439,915 which issued Apr. 3, 1984 or plate heaters can be secured along the surface of the manifold as shown in the present case.
Depending upon the type of material being molded, these manifolds may be heated to temperatures over 600.degree. F. Thus, because of the considerable mass of each manifold, it takes quite a while for them to cool down when the system is shut down. This often results in deterioration of the melt which remains in the melt passage in the manifold. This is due to the fact that many plastic melts are temperature sensitive over a period of time and will decompose and result in discolouration or carbonization. When the system is restarted, the deteriorated material is ejected into the cavities resulting in scrap products, or if the deterioration is more extreme, the system must be dissassembled and the melt passages cleaned out. At present, this problem can be avoided by flushing out the system with a very heat stable material on shut down, but this causes other problems when restarting.